High pressure discharge lamp

Abstract: An electric discharge lamp has a discharge vessel (11) including pair of opposing planar seals (14) each having a pair of major faces (14a, b) and a pair of minor side faces (14c, d). A containment shield surrounds the discharge vessel (11) and includes a glass sleeve (21) and a helically coiled wire (22) secured on the glass sleeve. The glass sleeve is pinched to ends of the minor side faces (14c, d) of the seals (14) only and substantially does not engage the major faces (14a, b) of the seal. The wire (22) is fixed around the sleeve (21) in an electrically floating manner, e.g. by clamping fit.

Description: The invention relates to a high pressure discharge lamp comprising: an outer envelope; a discharge vessel arranged within the outer envelope, the discharge vessel including a body portion enclosing a discharge space, a pair of opposing electrodes within said body portion between which a discharge is maintained during lamp operation, and a pair of opposing seals sealing the discharge vessel in a gas-tight manner, each of the seals having a pair of opposing major faces and a pair of minor side faces extending between the major faces; frame means for supporting the discharge vessel within the outer envelope and for electrically connecting the discharge vessel to a source of electric potential outside of the lamp envelope; and a light-transmissive sleeve disposed about the discharge vessel and having an end pinched to a said seal. Such a lamp has been publicly disclosed by Venture Lighting Company of Cleveland, Ohio as a metal halide lamp in 70 W and 100 W sizes. The sleeve and discharge vessel for this lamp are shown in Figure 1. The purpose of the containment sleeve 1 is to contain fragments of the discharge vessel 2 and prevent failure of the outer envelope (not shown) in the rare event of discharge vessel rupture. One end 3 of the sleeve is open while the other end 4 is pinched to both major faces of one of the press seals 5. The discharge vessel is of the formed body type in which the body portion 6 of the discharge vessel, which lies between the press seals and in which the discharge is maintained between discharge electrodes 7, has a precise elliptical or ovoidal shape. The fusing of the sleeve to a portion of the discharge vessel is advantageous because lamp manufacturing is simple and no additional metal parts are introduced into the lamp envelope. However, the containment was found to be insufficient. The wall thickness of the sleeve in the lamp of Figure 1 was 2 mm. In tests in which the discharge vessel was ruptured by a current surge, failure of the outer envelope was found to occur. Additionally, the sleeve construction is asymmetric in that the pinched end of the sleeve is totally closed whereas the other side is open. The lower side of the discharge vessel will thus have a significantly different temperature, and the lamp will have different photometrics, depending on whether the lamp orientation is base-up or base-down, which is undesirable. Also from EP 0.549.056-A1 a discharge lamp is known, in which a sleeve is held around the discharge vessel by means of clamping strips which are welded to a rod of the lamp frame. A coiled metal wire clampingly surrounds the sleeve. From EP 0.550.094-A2 a similar discharge lamp is known. The difference with the afore-said lamp is, that the sleeve is secured in that it is fused to the tipped-off exhaust tube of the discharge vessel. Although the construction of this lamp is reliable, the fixation of the sleeve to the discharge vessel at one sole area thereof does make the lamp sensitive to shocks. In the development of low wattage (35-100 W) metal halide lamps, particularly as replacements for incandescent and halogen lamps for interior and display lighting, the first lamps made were basically smaller versions of conventional higher wattage lamps. The discharge vessel was formed from a cylindrical tube of quartz glass with press seals at each end. The middle portion of the arc tube retained the circular cylindrical shape of the tube. The performance of these early low-wattage lamps was inferior to the efficacy (lm/W) and colour rendering index (CRI) values in the region previously established for higher wattage lamps (i.e., 150-400 W), especially in the smaller 35 W and 50 W sizes. It was found that luminous efficacy and colour rendering generally worsened as the size and wattage of the discharge vessel were reduced. Later efforts to improve the performance of low wattage lamps concentrated mainly on discharge vessel shaping and miniaturization of the end chambers in which the electrodes are located and the press seals. Discharge vessels with precise elliptical or ovoidal discharge spaces resulted from these efforts. These discharge vessels, such as the type shown in Figure 1, are of high quality but are more expensive than discharge vessels pressed from straight tubing. Shaping the formed body requires successive, time consuming glass working steps which are not required for straight-body discharge vessels. The market calls for more cost effective low-wattage designs which can safely be used in open fixtures. However, cost reduced lamps will only be commercially successful if the photometrics of the lamps are acceptable. For low-wattage lamps to be considered of "standard quality" the initial efficacy (after 100 hours) should be greater than about 80 lm/W for 100 watt lamps, greater than about 75 for 70 watt lamps and greater than about 65 for 50 watt lamps. The initial CRI should be greater than about 60 for each of these lamps. It is an object of the invention to provide a high pressure discharge lamp of the kind mentioned in the opening paragraph which is of a simple, less costly and reliable construction while providing commercially acceptable photometrics. According to the invention, this object is achieved in that a lamp of the type described in the opening paragraph is characterized in that: the sleeve is pinched to the press seal along only the minor, side faces thereof. By pinching the sleeve against only the minor, side faces of the press seal, both ends of the sleeve may remain open. Thus, the problems of asymmetry associated with pinching the sleeve along the major faces as in the prior art are obviated. Additionally, pinching the sleeve against the side faces, especially along the side faces of both seals, provides a sturdy construction in which clamping strips or clips can be eliminated. Thus, a simple, low cost, symmetric construction is obtained which overcomes the above-noted disadvantages in the prior art. In a favourable embodiment, the sleeve is pinched against only the end portions of the side faces. This axially captures the discharge vessel within the sleeve while minimizing contact between the sleeve and discharge vessel so that cracking of the sleeve due to differences in thermal expansion between the sleeve and the discharge vessel is avoided. According to another aspect of the invention, a helically coiled metal wire surrounds the glass sleeve and is fixed around this tube so as to be electrically floating. The coiled wire permits the thickness of the sleeve to be reduced so that similar containment capabilities can be achieved with a containment shield which has about half the weight of a sleeve used without such wire. In a favourable embodiment, the metal wire includes portions bent over the ends of the glass tube. The portions then axially secure the metal wire on the tube in a simple fashion. The wire may also have a clamping fit with the sleeve. In another embodiment of the invention, the lamp is a low wattage metal halide lamp in which the body portion of the discharge vessel between the end chambers is cylindrical, the discharge electrodes extend axially within the discharge vessel and include an electrode rod and coil overwind, and the end chambers of the discharge vessel are free of a heat conserving end coating. The term "low wattage" as used herein means a metal halide lamp having a rated wattage of about 100 W or less. It was a surprise to find that "standard quality" performance could be achieved in a low wattage metal halide with such a discharge vessel. This is because almost all commercially available metal halide lamps have a heat conserving end coating of, for example, zirconium oxide or aluminum oxide. Such coatings serve to increase the temperature of the discharge vessel in the area at which the fill constituents condense to improve lamp photometrics. Of known low-wattage lamps which are commercially available and which have a cylindrical, or "straight", body discharge vessel, none are free of such a coating. Elimination of the end coating results in significant cost savings and partially offsets the cost of providing the shield. It was another surprise to find that lumen maintenance was significantly improved without the end coating. These and other objects, features, and advantages of the invention will become apparent with reference to the following drawings and detailed description. Figure 1 shows a metal halide discharge vessel/sleeve assembly according to the prior art; Figure 2 illustrates a high pressure discharge lamp according to the invention, Figure 3 is an elevation of the discharge vessel of the lamp of Figure 2; Figure 4 is an end view of the discharge vessel of Fig. 2. Figure 2 shows a high pressure discharge lamp having a sealed outer envelope 10 in which a discharge vessel 11 is arranged. Frame means 17a, 17b shaped as conductive support rods extend from the stem 16 and are connected to lamp cap 19 outside the outer envelope and to respective ones of the discharge vessel feed-throughs 18 via conductive straps 17c, 17d. During lamp operation an electric potential is applied across the discharge vessel and a gas discharge is maintained between the discharge electrodes 15. The discharge vessel (Fig. 3) is formed from a length of straight circular-cylindrical tubing of quartz glass and includes opposing planar press seals 14, which seal the discharge vessel in a gas-tight manner. Between the press seals 14, the discharge vessel includes a central, tubular portion 12 of a constant circular cross-section and end chambers 13 of continuously reducing cross-section which result from the pressing of the seals 14. Each of the seals 14 has a pair of opposing major faces (14a, 14b) and a pair of minor, side faces (14c, 14d) extending between the major faces. The portion 12 further includes a tipped-off exhaust tube 12a. A discharge sustaining filling within the discharge vessel includes mercury, an inert gas and one or more metal halides. The electrodes 15 are conventional and include an electrode rod 15a with a coil wrap 15b. In order to contain fragments of the discharge vessel within the outer envelope upon explosion, a containment shield 20 surrounds the discharge vessel and includes a vitreous light-transmissive sleeve 21 and a length of helically coiled wire 22 about the sleeve. The sleeve may consist of, for example, hard glass or quartz glass. The sleeve 21 has an inner diameter, over a major portion of its length, which is only slightly larger than the largest width dimension between the side faces 14c, 14d, allowing the sleeve to be readily positioned over the discharge vessel. The ends of the sleeve are pinched against ends of the side faces of the press seals to axially capture the discharge vessel within the sleeve. This is accomplished by heating the ends of the sleeve opposite the minor seal faces 14c, 14d to its softening temperature and allowing it to just collapse onto the minor faces or by gently pressing the softened glass against the minor faces with suitable jaws. As a result of this, the sleeve includes indentations 21a which extend along the edge of the press seal side faces and portions 21b pressed against the end of the respective side faces (Fig. 4). With the sleeve pinched against the ends of the seal, which are the coolest part of the seal, less heat is conducted from the discharge vessel to the sleeve 21 than if the sleeve is fused across the major faces of the press seal as shown in the prior art lamp of Figure 1. Additionally, both sleeve ends remain open and are symmetrical, so that the temperature distribution of the arc tube will be substantially the same whether the arc tube is operated base-up or base-down, in contrast to the prior art lamp of Figure 1. Thus, the constructional advantages associated with pinching the sleeve to the press seals are retained while conduction of heat away from the discharge vessel is minimized. The wire 22 is fixed around the sleeve by its own clamping force and is electrically floating. Bent end portions 22a engage over the ends of the sleeve to further axially secure it on the sleeve. To achieve this, for example, resistance wire may be used, for example, of kanthal, tantalum molydenum, or stainless steel. In the lamp shown, molybdenum wire of 0.60 mm diameter is used, coiled with a pitch of 5 mm. The containment shield 20 is electrically isolated from the lamp frame because no metallic straps secure the sleeve to the conductive support rods 17 and neither the sleeve 21 nor the metal coiled wire 22 contact any portion of the metallic lamp frame. The above construction is attractive because the discharge vessel 11, sleeve 21, and wire 22 can be provided during lamp assembly as a completed sub-assembly. The sub-assembly is then easily connected to the frame means by welding the ends of the conductive feed-throughs 18 to the conductive support straps 17c, 17d. To test the effectiveness of the containment shield, the discharge vessel was made to explode by means of a current surge. The outer envelope had a wall thickness which varies over its surface from about 0.6 mm to about 1 mm. The outer envelope remained entirely undamaged during this, which proves that the construction of the lamp effectively protects the surrounding against the consequences of an explosion of the discharge vessel. Using the same tests, the prior art lamp of Figure 1 in which the sleeve was pinched to the major faces of the sleeve suffered breakage of the outer envelope even though its wall thickness was significantly greater, at about 2 mm. Additionally, the lamp was drop tested. None of the lamps according to the invention were found to fail. The influence of the sleeve construction on the photometric performance was determined by fabricating low wattage metal halide lamps having a straight-body discharge vessel according to Figure 3 and a sleeve construction according to Figure 2. Table I lists the results for a group of 100 W metal halide lamps having a fill including sodium iodide and scandium iodide in a mole ratio of NaI/ScI 3 of 19:1. The cylindrical portion of the discharge vessel had an internal diameter of about 6 mm. The distance between the electrode tips (dimension A in Figure 3), was about 13.5 mm. The cavity length (dimension B in Figure 3) was about 23 mm. Table II lists the results for a group of 70 W metal halide lamp having a fill including sodium iodide and scandium iodide in a mole ratio of NaI/ScI 3 of 19:1. The cylindrical portion of the discharge vessel was also about 6 mm. The arc gap was about 10 mm and the cavity length was about 17 mm. None of the Table I-II lamps had a heat conserving end coat about the end chambers. The outer envelopes had a gas fill of nitrogen, at a pressure of about 1 atmosphere during stable operation. Hrs Volts lm/W % lm/W CCT CRI 100 98 86 100 3982 60 1000 98 80 93 3986 59 2000 98 74 86 3907 60 5000 99 59 69 3812 58 Hrs Volts lm/W % lm/W CCT CRI 100 85 77 100 4116 57 1000 86 74 96 4058 57 2000 87 65 85 3895 55 5000 89 50 65 4201 51 At 100 hours, each of the above lamps meets the design goal of "standard quality" photometrics, i.e. a CRI of about 60 and lm/W of greater than about 80 for a 100 W lamp and greater than about 75 for a 75 W lamp. This was surprising because the discharge vessels did not have an end coat. There are no commercially available low wattage metal halide lamps having straight-body discharge vessel without an end coat. For the sake of comparison, a group of 100 W lamps having the same discharge vessel as those in Table I with a conventional Zr0 2 end coat and no sleeve had 100 hour values of 80 lm/W and CRI 60. One hundred watt lamps with the same discharge vessel with no end coat and no sleeve had 100 hour values of 75 lm/W and CRI 55. Thus, the sleeve design according to the invention (without an end coat) provides an improvement of 10 lm/W and a CRI increase of 5 over lamps without a sleeve or end coat and at least the same improvement as that of a standard end coat. While it is generally known that a sleeve can improve photometric performance, the extent of the improvement was surprising. For the 100 W and 70 W lamps of Tables I - II, the radial distance between the minor face 14c, 14d and outer wall of the cylindrical portion 12 of the discharge vessel is about 2-3 mm, which is achieved during the standard pressing of the seals 14. The sleeve is selected to fit closely to the minor faces so that it is also spaced only about 2-3 mm from the cylindrical portion. This close spacing is important to provide optimized heat conservation from the discharge vessel. The novel pinching along the minor faces permits of obtaining this close spacing with a simple construction which also minimizes conduction of heat away from the press seal end chamber areas. Additionally, it was a surprise to find in several test groups that lamps without a ZrO 2 end coat on the end chambers experienced improved lumen maintenance as compared to lamps with such an end coat. From tables I-II, the lumen maintenance at 5000 hours was 69% and 65%, respectively, for 100W and 70 W lamps as compared to a lumen maintenance of 52% for a 100W lamp with the same discharge vessel and no sleeve but with a ZrO 2 end coat. Thus, in addition to the cost savings and reduced spread in photometric parameters among lamps, eliminating the end coat can also lead to improved lumen maintenance.

Claims: A high pressure discharge lamp comprising an outer envelope (10), a vitreous discharge vessel (11) arranged within said outer envelope, said discharge vessel including a body portion enclosing a discharge space, a pair of opposing discharge electrodes (15) within said body portion between which a discharge is maintained during lamp operation, and a pair of opposing seals (14) sealing said discharge vessel in a gas-tight manner, each of said seals having a pair of opposing major faces (14a, b) and a pair of minor, side faces (14c, d) extending between said major faces, frame means (17a, b) for supporting said discharge vessel within said outer envelope and for electrically connecting said discharge vessel to a source of electric potential outside of said lamp envelope, and a light-transmissive sleeve (21) arranged about said discharge vessel and having an end pinched to one of said seals (14), characterized in that: said sleeve (21) is pinched to said seal (14) along only said minor, side faces (14c, d). A high pressure discharge lamp according to claim 1, wherein said sleeve (21) is pinched to both of said seals (14) along only said minor, side faces (14c, d). A high pressure discharge lamp according to claim 1 or 2, wherein said sleeve (21) is pinched to said minor, side faces (14c, d) only at its end portions. A high pressure discharge lamp according to claim 5, further comprising a helically coiled metal wire (22) coiled about said sleeve (21). A high pressure discharge lamp according to claim 4, wherein said coiled length of wire (22) includes bent end portions (22a) at each end thereof, said bent end portions being bent over respective opposing ends of said sleeve (21). A high pressure discharge lamp according to claim 4 or 5, wherein said helically coiled metal wire (22) has a clamping fit with said sleeve (21).

Inventors: KOWAL ALBERT, PRAGT HENRIKUS, VAN DER LEEUW BART, KOWAL, ALBERT, PRAGT, HENRIKUS, VAN DER LEEUW, BART
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1994
Main classification: H01J61
IPC codes: H01J61